On the host, each Linode uses two file descriptors for the console, one to /dev/null, one for each disk image, and one for each memory map the UML kernel opens.

I've observed Linodes consuming an average of 20-30 file descriptors on the host. There is a limit placed on each Linode on the host itself, but it is an insanely high value that's only there as a break for run-away Linodes (or other evil-doers).

Inside your Linode, you are free to use as many file descriptors as your UML kernel can handle (or /proc/sys/fs/file-max), which has a compiled in max of 1048576 open fd.